The high reliability goals typically set for communication switching systems are frequently met by implementing those systems with redundant hardware. For example, redundant switching networks can be included in such systems wherein an active network handles all traffic until the system detects a failure of that network. A standby network then becomes active and handles traffic while the failure is diagnosed and corrective measures are taken.
In one known duplicated switching system, digital words are conveyed among a number of communication units in time-multiplexed channels by an active time-multiplexed switch. The duplicate time-multiplexed switch is designated standby. However, each digital word transmitted from an originating communication unit to a destination communication unit via the active time-multiplexed switch is also transmitted via separate interface and communication link hardware through the standby time-multiplexed switch. Although digital words are transmitted through the standby time-multiplexed switch to the destination communication unit, those digital words are not used for further communication. In this known system, when a fault develops, for example, in the communication link hardware between one communication unit and the active time-multiplexed switch, all of the communication units, under the control of a central control, begin using the digital words from the other time-multiplexed switch for communication. Not only does this change require a good deal of coordination between the central control and the communication units but it also leaves the system vulnerable to the occurrence of a second fault of this type before the first one can be repaired resulting in a complete system outage.
In a second known duplicated switching system, digital words are conveyed among communication units in time-multiplexed channels by duplicate time-multiplexed switches and a select bit is included with each digital word conveyed. For example, a digital word representing a given speech sample and including a select bit of a first logic value may be conveyed by the first time-multiplexed switch while a digital word representing the same given speech sample but having a select bit of a second logic value is conveyed by the second time-multiplexed switch. Both time-multiplexed switches transmit the digital words to the destination communication unit. However, a logic circuit in the destination communication unit selects only those digital words having select bits of the first logic value to be used for further communication. When the switching system central control detects system faults affecting, for example, only the digital words transmitted by a given originating communication unit and conveyed by the first time-multiplexed switch, the central control controls the select bits of the digital words conveyed by the time-multiplexed switches such that destination communication units use only the digital words from the given originating communication unit that are conveyed via the second time-multiplexed switch. However, in applications where the reliability of communication is important, e.g., data communication, the time needed for the central control of such a system to detect a fault and to appropriately control the select bits is sufficiently long that complex error detection/correction or data retransmission methods must be used to recover from the errors that occur before such central control measures become effective. In view of the foregoing, a recognized problem in the art is the additional complexity required to achieve highly reliable communication through a duplicated switching system when the responsibility for fault recovery lies solely with the system central control.